[hotfix] Mention mTLS in SSL documentation page

[rmetzger]

Make it more explicit that Flink supports mTLS. This term doesn't get mentioned in the docs, but is a common search term for this concept.

[flinkbot]

CI report:

• 78c85b8180d2c0e133eb7218c3f39aba81073003 Azure: SUCCESS

Bot commands

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• @flinkbot run azure re-run the last Azure build

[rmetzger]

Thanks a lot!